Modern wind turbines comprise a plurality of wind turbine rotor blades, typically three blades, each blade today having a weight of up to 15 tons and a length of up to 65 meters.
In order to facilitate transportation of such large blades, it is envisaged that the blades are made from a plurality of blade sections, which are assembled at the erection site of the wind turbine. The blade sections may be connected in a joint zone when the blade is assembled. However, such joints may cause the blade to be structurally weakened in the area of the joint and, thus, there is a need for reinforcing the blade this area.
In use, the joint is subjected to bending and centrifugal loading caused by the rotation of the blade. Accordingly, the joint must be designed to cope with the shear stress and normal stress caused by these types of loading. Both types of stress in the joint are dependent on the blade section thickness, thus, increasing the thickness reduces the stress e.g. due to bending.
However, from an aerodynamic point of view, it is often preferred to minimise the thickness of the blade. Accordingly, the skilled person is faced with two contradictory principles: increasing the thickness improves the strength which however is undesirable from an aerodynamic viewpoint, and reducing the thickness is desirable from an aerodynamic point of view, but this reduces the strength of the joint.
In order to address the challenge, high strength materials such as fibre glass and carbon fibre are often used to achieve the desired strength and aerodynamic profile. Such materials are often fastened to each other by means of an adhesive such as a two-component epoxy adhesive.
In the case of bolted joints, metal bolts may take the load. However, high strength steel can take less stress than carbon fibre, and just considering the static loads, the bolts need to have approximately one and a half times the cross sectional area of the carbon fibre in order to be able to take the same load. Furthermore, due to fatigue loading, metal bolts would need an even greater cross sectional area. This, however, forces the skilled person to increase the thickness of the blade which, as stated above, is undesirable.
It is an object of an embodiment of the present invention to provide a sectional blade which has an increased strength in the area of the joint.
Moreover, it is an object of an embodiment of the present invention to provide a sectional blade which minimises any reduction in the aerodynamic performance in the area of the joint.